Studying the localization of proteins with conventional antibodies has greatly contributed to our understanding of the structure and function of neurons. However, conventional antibodies have several limitations that drastically limit their utility. Tissue must be fixed and permeabilized prior to staining and often the overlapping expression patterns of adjacent neurons are difficult to interpret because of the lack of contextual information. For these reasons, the precise subcellular localization patterns in vivo of the majority of neuronal proteins have not been well characterized. The purpose of the studies proposed in this grant is to develop genetically encoded probes that will allow the subcellular localization of neuronal proteins to be mapped in vivo and in real time with high fidelity. These probes consist of genetically encoded aptamers (intrabodies) that bind to endogenous neuronal proteins and are generated using the mRNA display system. Three different types of intrabodies will be generated: 1. Binders to individual cytoskeletal proteins that mark neuronal structures such as pre- and postsynaptic sites. 2. Binders to transmembrane proteins. These intrabodies will be modified to enable them to label either total protein or only protein that is present on the plasma membrane of the cell. 3. Binders to activated G-proteins. Intrabodies will be used to attach three types of molecules to endogenous target proteins: 1. Fluorescent molecules that can be used to report the localization of the protein. 2. proteins for measuring Ca++ concentration in the region around the protein. 3. proteins that are activated by light to produce depolarizing currents. Subcellular trafficking of proteins is crucial to virtually all neuronal functions, including establishment of synaptic connections, axon guidance and synaptic plasticity. Disruption of protein trafficking has been linked to such diseases as Alzheimer's disease and Parkinson's disease. Protein trafficking also plays a critical role in drug addiction. Intrabodies generated through RNA display will provide tools to map the subcellular localization of endogenous proteins with high fidelity, in vivo and in real time, which is not possible with current technology.